1. Field of the Invention
The present invention relates generally to electrochemical sensors that can be used for the quantification of a specific component or analyte in a liquid sample. Particularly, the present invention relates to a disposable electrochemical sensor for measuring urea (or BUN, Blood Urea Nitrogen) in biological fluids such as blood. More particularly, the present invention relates to a system for simultaneously determining creatinine and BUN-to-creatinine ratio in a single device. Still more particularly, the present invention relates to a device that can be employed to perform assays of urea, creatinine and blood urea nitrogen-to-creatinine ratio in a small volume liquid sample (around 1 μL).
2. Description of the Prior Art
In the liver, urea is synthesized from ammonia produced as a result of de-amination of amino acids. This biosynthetic pathway is the main means of disposal of excess nitrogen by the body. Therefore, the measurement of blood urea nitrogen (BUN) is one of the most commonly used screening tests for the evaluation of kidney function. High urea concentration in blood is toxic to the body. Malfunctioning kidneys will have difficulty in getting rid of the excess urea.
There are currently several hundred thousand people with endstage renal disease in the United States. These people require regular hemodialysis. Urea is generally accepted to be the best marker for evaluating the level of uremic toxins. Hemodialysis is a procedure that has been used as a means for reduction of urea in blood. Currently, most dialysis clinics use the simple index of time of dialysis to determine the adequacy of dialysis. Drawing blood to accurately measure the level of urea in the blood is done infrequently. During this procedure, blood is drawn and sent to a central laboratory for the measurement of urea. Turnaround times for these samples can be quite long.
It has long been desired to have a sensor which could monitor the progress of the dialysis procedure in “real-time”, thereby assuring that the procedure was complete and obviating the need for using clinical laboratories. A further enhancement of this general principle is a home monitor, which would allow at-home testing to monitor the peritoneal dialysis and home hemodialysis. Such a home monitor is similar, in principle, to devices used for blood glucose testing by diabetics. This device would require a blood droplet sample by using a finger prick.
There has been substantial efforts in the prior art to provide a sensor which would satisfy the above-mentioned needs, and various electrochemical sensors for detecting urea in body fluids have been proposed. These prior art devices included the use of potentiometric devices, specifically, the common pH, ammonium, or ammonia gas sensing electrodes. They also included a biosensor for urea by depositing a coating containing the enzyme urease immobilized over the interdigitated conductive members.
Other publications teach of the need for dialysis or blood urea monitoring, and various mechanical systems which could be used in such a system, albeit with different or unspecified urea detection (sensor) methods than those of the present invention. The prior approaches also describe various methods of enzyme (urease) immobilization to various substrates for detection of urea in blood or in dialysate fluid. For example, Cozzette et. al. (U.S. Pat. Nos. 5,466,575; 5,063,081) described a microfabricated BUN sensor, which is based on the potentiometric measurement of ammonium ions resulting from an enzymatic reaction. None of the previous approaches, however, describes a combination of disposable, small volume, mass fabricated sensors with dissolvable reagent matrix containing the enzyme urease.
The prior known sensors have not enabled the desired point-of-care blood test for blood urea, thereby assuring that the procedure was complete. In the absence of a reliable point-of-care blood test for blood urea, to determine whether the dialysis process is long enough to rid the excess urea, it will be desirable to have a sensor which can be used to monitor the progress of the treatment simply by measuring the blood urea concentration from a drop of blood obtained from a finger prick.
It is also well known that creatinine is a waste product derived from creatine and excreted by the kidneys. The analytical determination of creatinine in biological samples is a widely used and extremely important test for renal dysfunction. Measurements of creatinine in whole blood, serum or urine may also be used as indices in the diagnosis and treatment of other disorders such as muscular dystrophy and hypothyroidism. Thus, the creatinine assay has been widely recognized as having vital medical significance. Further, dietary changes have little, if any, influence on the creatinine concentration in blood and urine.
The BUN-to-creatinine ratio is an important index used by health professionals to predict what conditions may be causing abnormal BUN and creatinine levels and decreased kidney function. The normal values for the BUN-to-creatinine ratio are typically in the 10:1 to 20:1 range for persons over 12 months of age and up to 30:1 for infants less than 12 months of age. A high BUN value can indicate kidney injury or diseases, such as, diabetes, high blood pressure, kidney stone, or tumor. It can also be caused by reduced blood flow to the kidneys caused by dehydration or heart failure. Certain medications may also cause high BUN values.
High BUN-to-creatinine ratios occur with sudden kidney failure, which may be caused by conditions such as shock or severe dehydration. An obstruction in the urinary tract can also cause an elevated BUN-to-creatinine ratio. A very high BUN-to-creatinine ratio may be caused by bleeding in the digestive tract or respiratory tract.
On the other hand, a low BUN value may be caused by a diet very low in protein, malnutrition, or severe liver damage. Drinking excessive amounts of liquid may cause overhydration and lead to a low BUN value.
Low BUN-to-creatinine ratios may be associated with a diet low in protein, severe muscle injury called rhabdomyolysis, pregnancy, cirrhosis, or syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH sometimes occurs with lung disease, cancer, diseases of the central nervous system, and the use of certain medications.
Therefore, what is needed is a disposable sensor capable of measuring urea in a small volume of body fluid sample such as blood. What is also needed is a disposable sensor capable of measuring urea in a small volume of body fluid such as blood obtained by lancing the skin of a user. What is further needed is a system for determining the blood urea, creatinine and blood urea nitrogen-to-creatinine ratio in a small volume of body fluid using a single device.